Device for air cleaning

ABSTRACT

A device is provided for cleaning air from charged particles in an air flow duct. The device includes at least one precipitator panel including one or more precipitator units that are arranged so that their inlets have a large extension relative to the cross-sectional area of the air flow duct and a relatively small extension in the direction of the air flow through the units. The precipitator panels include at least two electrode elements preferably connected to a high voltage source and located such that all air in the air flow duct passes through the respective electrode elements.

This application is a national stage application of InternationalApplication No. PCT/SE98/01437, filed on Aug. 5, 1998.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a device for cleaning air fromelectrically charged particles (aerosols), said device including atleast one precipitator panel, said panel including at least oneprecipitator unit having at least two electrode elements or at least twogroups of electrode elements, said elements being located alternatelyrelative to each other by a an internal gap distance, said electrodeelements being suitably connected to respective terminals of a highvoltage source, said device being located in an air flow duct or inimmediate connection with an air flow duct.

PRIOR ART

Particle filters for use in ventilating applications or so called ductfilters are usually designed around mechanical so called barrierfilters. The separating capacity with regard to particle separationvaries widely for these filters depending on the structural design (thefilter class), i.e. coarse filters, fine filters and micro filters.Characterising for these filters are among other things an substantiallyincreasing pressure drop in relation to the ability to separate microparticles. This disadvantage gives rise to a need for powerful airtransporting fans, said fans having a high noise level and of courseunnecessary high energy consumption together with expensive installationcosts. Also, the increasing demands for improved indoor air and demandsfor clean ventilating ducts have increased the use of higher filterclasses.

Mechanical filters of electrically charged fibres, so called electretfilters, have initially better operation characteristics than othertypes of barrier filters. However, these characteristics are notoperationally stable and decrease eventually.

The use of the traditional electro filter technique, i.e. usingprecipitators of metallic electrode elements instead of mechanicalfilters has up to now not been successful to any higher degree. Thisdepends on high installation costs and complicated and expensiveservice. The recent development of the electro filter technique usingfilter cassettes of paper has up to now not been used in ventilatingduct applications since also this technique has its limitations,especially in such a demanding environment as ventilating ducts, saidenvironment having a temperature and a humidity that varies widely andan air flow velocity that is several times higher than in air cleaners,said air cleaners being the devices that the technique in questionbasically is developed for.

The reasons for said limitations are the following. The precipitatordesigned out of board (cellulose based material), i.e. high ohmicmaterial, is affected by dust that bridges the gap between adjacentelectrode elements, i.e. electrode elements connected to respectiveterminal of a high voltage source. This affection increases byincreasing air humidity and decreases dramatically the particleseparating capacity of electro filters. The bridging dust betweenadjacent electrode elements deflects namely the electrical charging fromthe surfaces of the electrode elements, the effect of this is that thepotential between said surfaces decreases and consequently that theparticle (aerosols) separation capacity decreases.

OBJECTS AND FEATURES OF THE INVENTION

The aim of the present invention is to eliminate said limitations andthus create a practical and economical alternative for a new type ofventilating filter or duct filter of electrostatic character. In thisconnection the expression duct filter defines, apart from filters fordomestic ventilation, i.e. filters for supply air and/or exhaust air,also other applications, e.g. filters for coupes of motor cars, i.e.integrated in the ventilating device of the motor car, and also otherindustrial applications having relatively high air flow velocities. Itis of course also possible to use the technique in other circumstances,e.g. when designing air cleaners, cooker hoods etc. The most importantadvantages with the new type of filter are the high separating capacityof micro particles also in combination with simultaneous separation ofheavier particles, this being effected by an extremely low pressure dropand simple service using a vacuum cleaner or replacement of the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic perspective view of a first embodiment of a deviceaccording to the present invention, a portion of a ventilating ductbeing cut away;

FIG. 2 is a schematic perspective view of a second embodiment of adevice according to the present invention, a portion of a ventilatingduct being cut away;

FIG. 3 is a schematic perspective view of a third embodiment of a deviceaccording to the present invention, a portion of a ventilating ductbeing cut away;

FIG. 4 shows schematically the possibility of multiple design of theembodiment according to FIG. 2;

FIG. 5 shows schematically the possibility of multiple design of theembodiment according to FIG. 3;

FIG. 6 is a diagram showing the relation between the area enlargement Xand the gap distance “d”;

FIG. 7 is a diagram showing the relation between the depth “b” of theprecipitator and the gap distance “d”;

FIG. 8 is a schematic perspective view of a bobbin where band shapedelectrode elements are wound around said bobbin;

FIG. 9 is a schematic perspective view of an alternative design of abobbin compared to FIG. 8;

FIG. 10 is a schematic front view of a fourth embodiment of the deviceaccording to the present invention;

FIG. 11 is a schematic side view of the embodiment according to FIG. 10;and

FIG. 12 is a schematic top view of the embodiment according to FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS THE INVENTION

The present invention will below be described more in detail, referencebeing made to the accompanying figures.

FIG. 1 shows schematically the first embodiment of the presentinvention. A precipitator in the shape of panel 11 is provided in an airflow duct 09, the inlet area A of said panel 11 being located inclinedacross the air flow duct 09, seen in the air flow direction through theduct, and in such a way that essentially all air transport takes placethrough the precipitator panel 11. This can also be expressed as theprecipitator panel 11 is inclined relative to the air flow directionthrough the device.

The precipitator panel 11 may according to the characterising featuresinclude one or more units, i.e. independent precipitators, each unitconsisting of at least two electrode elements 01, 02 or at least twogroups of electrode elements preferably connected to respectiveterminals of a high voltage source HVU, said units being pervious to theair flow and having a gap distance “d” between adjacent electrodeelements 01, 02.

The depth “b” of the precipitator 11, i.e. the shortest way for the airflow to pass through said precipitator 11, is essentially smaller thanthe extension of the inlet area A of the precipitator 11.

According to what is shown in FIG. 1 the precipitator panel 11 maypreferably consist of one or more precipitator units designed inaccordance with patent application PCT/SE97/00956 or similarembodiments, i.e. essentially circular units or significantly roundedunits comprising at least two band-like electrode elements 01, 02arranged at a gap distance “d” relative to each other, said elementsbeing provided to circle several times around an axis, or a bobbin body08 substituting said axis, and connected in a suitable way to respectiveterminals of a high voltage source HVU.

In such an embodiment it is essential that the space between the activepanel portion, i.e. the precipitator unit, and the inner walls of theair flow duct are impervious to air flow. This is effected in FIG. 1 bymeans of the cover 20 of the precipitator panel having essentiallyrectangular or square shape.

Winding of the electrode elements 01, 02 around a bobbin body 08 ofrectangular or square cross section, said body having significantlyrounded edges as shown in FIG. 8, brings about a precipitator unithaving a correspondingly shaped inlet area and creates a rounded versionof the precipitator unit, said version having somewhat larger fillingfactor than a circular precipitator unit. However, the disadvantage isdecreased mechanical stability and this must be considered for eachapplication. In FIG. 9 is shown an improved bobbin body as regardsmechanical stability and the inherent electrical stability andsimultaneously better filling factor than the corresponding circularbobbin body, said bobbin body of FIG. 9 having at last two differentradii and hence the band elements 01, 02 during substantially theirentire length have a certain curvature. It is of course also possible todesign the active panel portion to have a rectangular inlet area, i.e.with electrode elements 01, 02 being essentially parallel to each other.However, such a design is not equally stable neither from mechanicalaspect nor from electrical aspect. The electrode elements 01, 02 have adeformation tendency when parameters relating to humidity/heat changeand hence there is a risk for short-circuit between said elements.

FIG. 2 shows an alternative embodiment of the present invention. Seenperpendicular to the air flow direction through the duct twoprecipitator panels 11 are located across the cross section of the airflow duct in such a way that all air transport takes place through theprecipitator panels. The precipitator panels 11 are inclined across theair flow duct 09, said panels being joined at a downstream edge, thuscreating a V-shaped precipitator unit.

FIG. 3 shows another embodiment having two precipitator panels 11arranged substantially parallel to each other and inclined relative tothe air flow direction through the duct 09. A suitable filling surface111, impervious to the air blow, is arranged in order to make all airtransport to take place through the respective precipitator panel 11.

FIG. 4 and 5 schematically show the possibility of multiple design ofthe embodiments shown in FIGS. 2 and 3.

The aggregated inlet area A_(tot), i.e. the sum of all active surfaces,pervious to air flow, of the precipitator panels of the device shouldaccording to the present invention be sufficient large to guarantee, incombination with the gap distance “d” and the panel depth “b”,essentially invariable operational features within a broad spectrum ofchanges as regards operative conditions, i.e. varying air humidity andtemperature, also after pollution.

Such design has turned out to be possible since at sufficient low airflow velocity through the precipitator, in combination with relativelysmall gap distance “d” between adjacent electrode elements 01, 02 of theprecipitator, dust accumulation takes place over the edge sections ofthe electrode elements, i.e. the inlet surface of the precipitator. Bydecreasing gap distance “d” there is a decreased migration tendency forheavy dust. This, as described above, to an essential degree decreasesthe dust influence upon the separation capacity of the precipitator, ofcourse with disregard to the dust influence that arises across the edgesections of the respective electrode elements 01, 02 at the inlet areaof the precipitator.

Therefore, the device according to the present invention should bedesigned on one hand with regard to the area enlargement X, i.e. thetotal inlet area A_(tot) in relation to the cross section area of theair flow duct 09, as a function of the gap distance “d” between theelectrode elements 01, 02 and on the other hand as a function of thedepth “b” of the precipitator, i.e. the shortest way for air flowthrough the panel.

The area enlargement factor X as a function of the gap distance “d”should according to the characterising claims be greater than a smallestvalue, and preferably greater than a preferred value according to thediagram of FIG. 6. The largest panel depth “b” should not exceed 10 cmand should as a preferred value and as a function of the gap distance“d” be within the shaded area according to FIG. 7.

As is evident from FIGS. 6 and 7 and standardised dimensions ofventilating ducts up to 600×600 mm it is the question of relativelylarge mechanical structures. In laboratory tests it has turned out thatwith a gap distance “d” up to 4 mm and demands for mechanical and hencealso electrical stability a preferred design for the precipitator panels11 has band-like electrode elements 01, 02 of thin band-like materialcircled several times and preferably coated by a damp-resistant film.However, also other materials, both conductive, semi-conductive ordissipative may be used as well as other ways to design precipitatorunits. Of course there is no prevention from adapting the precipitatorpanels 11 to 300×300 mm ventilating ducts and also use such precipitatorpanels in ducts having a 600×600 mm cross section if the 600×600 mm ductis divided into four 300×300 mm equal portions. It is also preferredthat the air flow duct where the device according to the presentinvention is to be located is area enlarged and thus the disposable airflow area is increased and thereby in practice larger area enlargementfactor X may be achieved than otherwise would have been possible.

The embodiment shown in FIGS. 10-12 relate to a supply air terminaldevice comprising an air flow duct 09′ having circular cross section anda transition space 12′, preferably located in the wall area of a room.As is most clearly evident from FIG. 12 the transition space 12′ has asubstantially larger cross section area, seen in direction of the airflow, than the air flow duct 09′. Preferably the front side of thetransition space is rounded. However, the air flow duct 09′ and thetransition space 12′ may of course have other cross sections than thoseshown in FIGS. 10-12.

In the air flow duct 09′ an ionisation device 10′ may be provided inorder to ionise the air flowing in the duct 09′, said ionisation device10′ in a known way being connected to a high voltage source (not shown).In the transition space 12′ a precipitator panel 11′ is located, saidpanel 11′ being inclined relative to the longitudinal direction of theair flow duct 09′, i.e. the direction of the air flow itself. In aconventional way the electrode elements of the precipitator panel 11′are connected to respective terminals of a high voltage source (notshown). Preferably the precipitator is of the type described above inconnection with FIG. 1. The precipitator panel 11′ is arranged in such away in the transition space 12′ that essentially all air transport takesplace through the precipitator panel 11′. Consequently at least aportion of the front side of the transition space 12′ is pervious to airto emit into the room the air that has passed the precipitator panel.The reason why the cross section area of the transition space 12′ isconsiderably larger than the cross section area of the air flow duct 09′is that the velocity of the air should be decreased before said air isemitted into the room. Otherwise people that are present in the room mayexperience an air draught from the device.

Contrary to mechanical filters designed according to different filterclasses the filter system dimensioned and designed in accordance withthe present invention may operate simultaneously as both coarse filterand micro filter. Such design rely upon a new knowledge of a practicalpossibility of considerably oversizing the precipitator units includedin a device, this being effected by designing the total inlet area ofthe precipitator panels several times larger than the cross section areaof the duct 09 (large area enlargement factor X) and decreasing gapdistance “d”.

In practice increasing area enlargement factor X decreases the air flowvelocity through respective precipitator unit. Within a broad range ofpossible potential decrease between the electrodes 01, 02 this does notaffect the separation capacity of said precipitator unit. Decreasing gapdistance “d” has turned out to decrease or prevent the migration of thebridging dust between adjacent electrode elements 01, 02, this in itsturn prevents potential decrease between said elements.

It is of course also possible to arrange so called cascade systems, i.e.two or more corresponding filter systems mounted subsequently after eachother seen in the direction of the air flow in the channel.

An upper limit for the area enlargement factor X does not exist and thepossibility of higher X-values for a certain given ventilating duct isincreasing, among other things through decreasing gap distance “d” anddecreasing band width of the electrode elements 01, 02.

The invention defined in the claims is not limited to any specialmaterial for the electrode elements 01, 02 of the precipitator butprecipitators of high-ohmic, including also dissipative, material ispreferred.

Preferably cellulose based material may be used, especially suchmaterial being provided with an extremely thin coating of plastic filmas a protection against damp.

The charging of the particles may be effected in a previously known wayupstream of the precipitator panels 11 or before the air is transportedthrough the air flow duct or in some other way.

The embodiments according to the present invention may rather easily beprovided with a device for removal (vacuum cleaning) of the collecteddust, this of course further increasing the operational reliability ofthe device and its service intervals.

For certain applications, e.g. car coupe filters, it may be suitable,and due to the access to a forceful air blow from the motion of the car,to arrange for a reversed air flow through the precipitator panel andthus blow away the collected dust out in the free air, this beingpossible if an air flow is used by the reversed action that is severaltimes more powerful than the designed operational condition.

Suitable gap distance “d” should for car coupe filters be less than 2mm. The area enlargement factor X should be higher than 4. By demand fora low volume this affects the depth of the precipitator panels 11.Suitable panel depth is less than 3.5 cm and preferably less than 2 cm.

Due to the relatively high enlargement factor X that is designed forthis invention the dust collection will take place on the top of theinlet area of respective precipitator and only micro dust will adhere tothe plane surfaces of the electrode elements 01 or 02. By decreasing gapdistance “d” the deposition of dust upon the inlet area of theprecipitator increases proportionally. Thereby it is both simple andeffective to remove dust collection by having the vacuum cleaner nozzleto sweep over the precipitator panels and the inlet areas respective.

The precipitator panels 11 are designed to be located in ventilatingducts as is shown in FIGS. 1 to 3 and 4, 5. The air flow may be effectedby mechanical fans provided in the ventilating ducts or in some otherway, e.g. through natural draught.

Nothing prevents that precipitator panels are arranged in accordancewith the principles of FIGS. 1-5, although the precipitators being in aseparate casing of preferably cellulose based material, said casinghaving such external dimensions that they are directly adapted forstandardised dimensions of ventilating ducts. The advantage is a simpleand hygienic handling including the possibility of incorporation of theentire device in connection with exchange, especially if there is a riskthat contaminated dust is separated in the device.

Such a device may of course also include ionisation chambers having thewalls of the duct as target electrode and an ionisation source accordingto previously known embodiments, the entire system being connected to ahigh voltage source in a suitable way.

What is claimed is:
 1. Device for cleaning air from electrically chargedparticles (aerosols), said device including at least one precipitatorpanel, said panel including at least one precipitator unit having atleast two electrode elements or at least two groups of electrodeelements, said elements being located alternately relative to each otherby an internal gap distance (d), said electrode elements being suitablyconnected to respective terminals of a high voltage source, said devicebeing located in an air flow duct or in immediate connection with an airflow duct, characterized in that the main plane of the precipitatorpanel is inclined relative to the direction of the air flow through thedevice, and that the precipitator panel is arranged in such a way thatessentially all air transport takes place though said precipitator panelsaid at least one precipitator panel comprising a circular or roundedprecipitator unit consisting of at least two band-shaped electrodeelements arranged to circle several times around an axis or a bobbinbody at an internal gap distance (d) relative to each other, saidelectrode elements being connected to a respective terminal of a highvoltage source, said precipitator unit being recessed in the cover ofsaid precipitator panel.
 2. Device according to claim 1, characterizedin that the device includes at least two precipitator panels in a commonmain plane.
 3. Device according to claim 1, characterized in that thedevice includes at least two precipitator panels in different mainplanes, said panels being joined at one edge.
 4. Device according toclaim 1, characterized in that the device includes at least twoprecipitator panels in different main planes, said panels being parallelto each other.
 5. Device according to any of claim 1, characterized inthat the precipitator panel is located in a space in immediateconnection with the air flow duct, said space having a substantiallylarger cross section area than the air flow duct seen in the directionof the air flow through the device.
 6. Device according to claim 1,characterized in the depth (b) of the precipitator panel is dimensionedin relation to the gap distance (d) between adjacent electrode elementsand depth (b) being less than 10 cm. when distance “d” is 4 mm. and lessthan 8 cm. when distance “d” is 2.4 mm., with the maximum distance “b”comprising an intermediate value increasing in straight-linerelationship between 8 and 10 when distance “d” is between 2.4 mm. and4.0 mm.
 7. A device according to claim 6, wherein said depth “b” is lessthan 5 cm. when said distance “d” is approximately 0 mm, and the maximumvalue of distance “d” comprises an intermediate value increasing instraight-line relationship between 5 cm. and 8 cm. when said distance“d” is between approximately 0 mm. and 2.4 mm.
 8. Device according toclaim 1, characterized in that an area enlargement factor (X) isdimensioned in relation to the gap distance (d) between adjacentelectrode elements said area enlargement factor being at least 1.1 when“d” is about 0.3 mm. and at least 2.7 when “d” is 4 mm., with theminimum value for “X” comprising an intermediate value increasing instraight-line relationship between 1.1 and 2.7 when “d” is between 0.3mm. and 4 mm.
 9. A device according to claim 8, wherein said areaenlargement factor “X” is at least 2.0 when said distance “d”approximates 0 mm., said “X” is at least 5.2 when “d” is 4 mm., and theminimum value for “X” comprising an intermediate value increasing instraight-line relationship between 2 and 5.2 when “d” is between 0 mm.and 4 mm.
 10. Device according to claim 1, characterized in thatcellulose based material is used for the electrode elements, saidmaterial being coated by a damp resistant film.